How can I make WPF ScrollViewer middle-click-scroll?

I have found how to achieve this using 3 mouse events (MouseDown, MouseUp, MouseMove). Their handlers are attached to the ScrollViewer element in the xaml below:

<Grid>
    <ScrollViewer MouseDown="ScrollViewer_MouseDown" MouseUp="ScrollViewer_MouseUp" MouseMove="ScrollViewer_MouseMove">
            <StackPanel x:Name="dynamicLongStackPanel">

            </StackPanel>
    </ScrollViewer>
    <Canvas x:Name="topLayer" IsHitTestVisible="False" />
</Grid>

It would be better to write a behaviour instead of events in code-behind, but not everyone has the necessary library, and also I don't know how to connect it with the Canvas.

The event handlers:

    private bool isMoving = false;                  //False - ignore mouse movements and don't scroll
    private bool isDeferredMovingStarted = false;   //True - Mouse down -> Mouse up without moving -> Move; False - Mouse down -> Move
    private Point? startPosition = null;
    private double slowdown = 200;                  //The number 200 is found from experiments, it should be corrected



    private void ScrollViewer_MouseDown(object sender, MouseButtonEventArgs e)
    {
        if (this.isMoving == true) //Moving with a released wheel and pressing a button
                this.CancelScrolling();
        else if (e.ChangedButton == MouseButton.Middle && e.ButtonState == MouseButtonState.Pressed)
        {
            if (this.isMoving == false) //Pressing a wheel the first time
            {
                this.isMoving = true;
                this.startPosition = e.GetPosition(sender as IInputElement);
                this.isDeferredMovingStarted = true; //the default value is true until the opposite value is set

                this.AddScrollSign(e.GetPosition(this.topLayer).X, e.GetPosition(this.topLayer).Y);
            }
        }
    }

    private void ScrollViewer_MouseUp(object sender, MouseButtonEventArgs e)
    {
        if (e.ChangedButton == MouseButton.Middle && e.ButtonState == MouseButtonState.Released && this.isDeferredMovingStarted != true)
            this.CancelScrolling();
    }

    private void CancelScrolling()
    {
        this.isMoving = false;
        this.startPosition = null;
        this.isDeferredMovingStarted = false;
        this.RemoveScrollSign();
    }

    private void ScrollViewer_MouseMove(object sender, MouseEventArgs e)
    {
        var sv = sender as ScrollViewer;

        if (this.isMoving && sv != null)
        {
            this.isDeferredMovingStarted = false; //standard scrolling (Mouse down -> Move)

            var currentPosition = e.GetPosition(sv);
            var offset = currentPosition - startPosition.Value;
            offset.Y /= slowdown;
            offset.X /= slowdown;

            //if(Math.Abs(offset.Y) > 25.0/slowdown)  //Some kind of a dead space, uncomment if it is neccessary
            sv.ScrollToVerticalOffset(sv.VerticalOffset + offset.Y);
            sv.ScrollToHorizontalOffset(sv.HorizontalOffset + offset.X);
        }
    }

If to remove the method calls AddScrollSign and RemoveScrollSign this example will work. But I have extended it with 2 methods which set scroll icon:

    private void AddScrollSign(double x, double y)
    {
        int size = 50;
        var img = new BitmapImage(new Uri(@"d:\middle_button_scroll.png"));
        var adorner = new Image() { Source = img, Width = size, Height = size };
        //var adorner = new Ellipse { Stroke = Brushes.Red, StrokeThickness = 2.0, Width = 20, Height = 20 };

        this.topLayer.Children.Add(adorner);
        Canvas.SetLeft(adorner, x - size / 2);
        Canvas.SetTop(adorner, y - size / 2);
    }

    private void RemoveScrollSign()
    {
        this.topLayer.Children.Clear();
    }

Example of icons:

And one last remark: there are some problems with the way Press -> Immediately Release -> Move. It is supposed to cancel scrolling if a user clicks the mouse left button, or any key of keyboard, or the application looses focus. There are many events and I don't have time to handle them all.

But standard way Press -> Move -> Release works without problems.

vorrtex posted a nice solution, please upvote him!

I do have some suggestions for his solution though, that are too lengthy to fit them all in comments, that's why I post a separate answer and direct it to him!

You mention problems with Press->Release->Move. You should use MouseCapturing to get the MouseEvents even when the Mouse is not over the ScrollViewer anymore. I have not tested it, but I guess your solution also fails in Press->Move->Move outside of ScrollViewer->Release, Mousecapturing will take care of that too.

Also you mention using a Behavior. I'd rather suggest an attached behavior that doesn't need extra dependencies.

You should definately not use an extra Canvas but do this in an Adorner.

The ScrollViewer itsself hosts a ScrollContentPresenter that defines an AdornerLayer. You should insert the Adorner there. This removes the need for any further dependency and also keeps the attached behavior as simple as IsMiddleScrollable="true".